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From: Alex =?utf-8?Q?Benn=C3=A9e?= <alex.bennee@linaro.org>
To: Aaron Lindsay <aaron@os.amperecomputing.com>
Subject: Re: plugins: Missing Store Exclusive Memory Accesses
Date: Thu, 21 Oct 2021 13:28:32 +0100
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Aaron Lindsay <aaron@os.amperecomputing.com> writes:

> On Oct 20 18:54, Alex Benn=C3=A9e wrote:
>> Have you got a test case you are using so I can try and replicate the
>> failure you are seeing? So far by inspection everything looks OK to me.
>
> I took some time today to put together a minimal(ish) reproducer using
> usermode. The source files used are below, I compiled the test binary on =
an
> AArch64 system using:
>
> $ gcc -g -o stxp stxp.s stxp.c
>
> Then built the plugin from stxp_plugin.cc, and ran it all like:
>
> qemu-aarch64 \
>     -cpu cortex-a57 \
>     -D stxp_plugin.log \
>     -d plugin \
>     -plugin 'stxp_plugin.so' \
>     ./stxp
>
> I observe that, for me, the objdump of stxp contains:
> 000000000040070c <loop>:
>   40070c:   f9800011    prfm    pstl1strm, [x0]
>   400710:   c87f4410    ldxp    x16, x17, [x0]
>   400714:   c8300c02    stxp    w16, x2, x3, [x0]
>   400718:   f1000652    subs    x18, x18, #0x1
>   40071c:   54000040    b.eq    400724 <done>  // b.none
>   400720:   17fffffb    b   40070c <loop>
>
> But the output in stxp_plugin.log looks something like:
> 	Executing PC: 0x40070c
> 	Executing PC: 0x400710
> 	PC 0x400710 accessed memory at 0x550080ec70
> 	PC 0x400710 accessed memory at 0x550080ec78
> 	Executing PC: 0x400714
> 	Executing PC: 0x400718
> 	Executing PC: 0x40071c
> 	Executing PC: 0x400720
>
> From this, I believe the ldxp instruction at PC 0x400710 is reporting two
> memory accesses but the stxp instruction at 0x400714 is not.

This is fascinating but I can't replicate your results. I get the
following pattern:

  Executing PC: 0x400910=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=
=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=
=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20
  Executing PC: 0x400914=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=
=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=
=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20
  PC 0x400914 accessed memory at 0x55007fffd0=20
  PC 0x400914 accessed memory at 0x55007fffd8=20=20=20=20=20=20=20=20=20=20=
=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=
=20=20=20=20=20=20=20=20
  Executing PC: 0x400918=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=
=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=
=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20
  PC 0x400918 accessed memory at 0x55007fffd0=20
  PC 0x400918 accessed memory at 0x55007fffd8=20=20=20=20=20=20=20=20=20=20=
=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=
=20=20=20=20=20=20=20=20
  PC 0x400918 accessed memory at 0x55007fffd0=20=20=20=20=20=20=20=20=20=20=
=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=
=20=20=20=20=20=20=20=20
  PC 0x400918 accessed memory at 0x55007fffd8=20=20=20=20=20=20=20=20=20=20=
=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=
=20=20=20=20=20=20=20=20
  Executing PC: 0x40091c=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=
=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=
=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20
  Executing PC: 0x400920=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=
=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=
=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20
  Executing PC: 0x400924=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=
=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=
=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20
  Executing PC: 0x400910=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=
=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=
=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20
  Executing PC: 0x400914=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=
=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=
=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20
  PC 0x400914 accessed memory at 0x55007fffd0=20=20=20=20=20=20=20=20=20=20=
=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=
=20=20=20=20=20=20=20=20
  PC 0x400914 accessed memory at 0x55007fffd8=20=20=20=20=20=20=20=20=20=20=
=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=
=20=20=20=20=20=20=20=20
  Executing PC: 0x400918=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=
=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=
=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20
  PC 0x400918 accessed memory at 0x55007fffd0=20=20=20=20=20=20=20=20=20=20=
=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=
=20=20=20=20=20=20=20=20
  PC 0x400918 accessed memory at 0x55007fffd8=20=20=20=20=20=20=20=20=20=20=
=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=
=20=20=20=20=20=20=20=20
  PC 0x400918 accessed memory at 0x55007fffd0=20=20=20=20=20=20=20=20=20=20=
=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=
=20=20=20=20=20=20=20=20
  PC 0x400918 accessed memory at 0x55007fffd8=20=20=20=20=20=20=20=20=20=20=
=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=
=20=20=20=20=20=20=20=20
  Executing PC: 0x40091c=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=
=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=
=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20
  Executing PC: 0x400920=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=
=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=
=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20
  Executing PC: 0x400924=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=
=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=
=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20
  Executing PC: 0x400910=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=
=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=
=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20
  Executing PC: 0x400914=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=
=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=
=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20
  PC 0x400914 accessed memory at 0x55007fffd0=20=20=20=20=20=20=20=20=20=20=
=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=
=20=20=20=20=20=20=20=20
  PC 0x400914 accessed memory at 0x55007fffd8=20=20=20=20=20=20=20=20=20=20=
=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=
=20=20=20=20=20=20=20=20
  Executing PC: 0x400918=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=
=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=
=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20
  PC 0x400918 accessed memory at 0x55007fffd0=20=20=20=20=20=20=20=20=20=20=
=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=
=20=20=20=20=20=20=20=20
  PC 0x400918 accessed memory at 0x55007fffd8=20=20=20=20=20=20=20=20=20=20=
=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=
=20=20=20=20=20=20=20=20
  PC 0x400918 accessed memory at 0x55007fffd0=20
  PC 0x400918 accessed memory at 0x55007fffd8=20=20=20=20=20=20=20=20=20=20=
=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=
=20=20=20=20=20=20=20=20
  Executing PC: 0x40091c=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=
=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=
=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20
  Executing PC: 0x400920=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=
=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=
=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20
  Executing PC: 0x400924=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=
=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=
=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20
  Executing PC: 0x400910=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=
=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=
=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20
  Executing PC: 0x400914=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=
=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=
=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20
  PC 0x400914 accessed memory at 0x55007fffd0=20=20=20=20=20=20=20=20=20=20=
=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=
=20=20=20=20=20=20=20=20
  PC 0x400914 accessed memory at 0x55007fffd8=20=20=20=20=20=20=20=20=20=20=
=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=
=20=20=20=20=20=20=20=20
  Executing PC: 0x400918=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=
=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=
=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20
  PC 0x400918 accessed memory at 0x55007fffd0=20
  PC 0x400918 accessed memory at 0x55007fffd8=20=20=20=20=20=20=20=20=20=20=
=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=
=20=20=20=20=20=20=20=20
  PC 0x400918 accessed memory at 0x55007fffd0=20=20=20=20=20=20=20=20=20=20=
=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=
=20=20=20=20=20=20=20=20
  PC 0x400918 accessed memory at 0x55007fffd8=20

It's a bit clearer if you use the contrib/execlog plugin:

  ./qemu-aarch64 -plugin contrib/plugins/libexeclog.so -d plugin  ./tests/t=
cg/aarch64-linux-user/stxp

  0, 0x400910, 0xf9800011, "prfm pstl1strm, [x0]
  0, 0x400914, 0xc87f4410, "ldxp x16, x17, [x0]", load, 0x55007fffd0, load,=
 0x55007fffd8=20
  0, 0x400918, 0xc8300c02, "stxp w16, x2, x3, [x0]", load, 0x55007fffd0, lo=
ad, 0x55007fffd8, store, 0x55007fffd0, store, 0x55007fffd8=20
  0, 0x40091c, 0xf1000652, "subs x18, x18, #1"
  0, 0x400920, 0x54000040, "b.eq #0x400928"
  0, 0x400924, 0x17fffffb, "b #0x400910"
  0, 0x400910, 0xf9800011, "prfm pstl1strm, [x0]
  0, 0x400914, 0xc87f4410, "ldxp x16, x17, [x0]", load, 0x55007fffd0, load,=
 0x55007fffd8=20
  0, 0x400918, 0xc8300c02, "stxp w16, x2, x3, [x0]", load, 0x55007fffd0, lo=
ad, 0x55007fffd8, store, 0x55007fffd0, store, 0x55007fffd8=20
  0, 0x40091c, 0xf1000652, "subs x18, x18, #1"
  0, 0x400920, 0x54000040, "b.eq #0x400928"
  0, 0x400924, 0x17fffffb, "b #0x400910"
  0, 0x400910, 0xf9800011, "prfm pstl1strm, [x0]
  0, 0x400914, 0xc87f4410, "ldxp x16, x17, [x0]", load, 0x55007fffd0, load,=
 0x55007fffd8=20
  0, 0x400918, 0xc8300c02, "stxp w16, x2, x3, [x0]", load, 0x55007fffd0, lo=
ad, 0x55007fffd8, store, 0x55007fffd0, store, 0x55007fffd8=20
  0, 0x40091c, 0xf1000652, "subs x18, x18, #1"
  0, 0x400920, 0x54000040, "b.eq #0x400928"
  0, 0x400924, 0x17fffffb, "b #0x400910"
  0, 0x400910, 0xf9800011, "prfm pstl1strm, [x0]
  0, 0x400914, 0xc87f4410, "ldxp x16, x17, [x0]", load, 0x55007fffd0, load,=
 0x55007fffd8=20
  0, 0x400918, 0xc8300c02, "stxp w16, x2, x3, [x0]", load, 0x55007fffd0, lo=
ad, 0x55007fffd8, store, 0x55007fffd0, store, 0x55007fffd8=20
  0, 0x40091c, 0xf1000652, "subs x18, x18, #1"
  0, 0x400920, 0x54000040, "b.eq #0x400928"
  0, 0x400924, 0x17fffffb, "b #0x400910"
  0, 0x400910, 0xf9800011, "prfm pstl1strm, [x0]
  0, 0x400914, 0xc87f4410, "ldxp x16, x17, [x0]", load, 0x55007fffd0, load,=
 0x55007fffd8=20
  0, 0x400918, 0xc8300c02, "stxp w16, x2, x3, [x0]", load, 0x55007fffd0, lo=
ad, 0x55007fffd8, store, 0x55007fffd0, store, 0x55007fffd8=20
  0, 0x40091c, 0xf1000652, "subs x18, x18, #1"
  0, 0x400920, 0x54000040, "b.eq #0x400928"
  0, 0x400924, 0x17fffffb, "b #0x400910"

Although you can see stxp looks a bit weird on account of the loads it
does during the cmpxchng. So consider me stumped. The only thing I can
thing of next is to see how closely I can replicate your build
environment.

--=20
Alex Benn=C3=A9e